WO2023139868A1 - Porous membrane, porous membrane laminate, and production method for porous membrane - Google Patents

Abstract

A porous membrane according to the present disclosure is mainly composed of polytetrafluoroethylene, wherein a 1st Run melting curve obtained by differential scanning calorimetry at a heating rate of 10°C/minute has an endothermic peak in a range of 300°C to 360°C inclusive, and the difference between the onset temperature and the endset temperature of the endothermic peak is not more than 20°C.

Description

Porous membrane, porous membrane laminate, and method for producing porous membrane

The present disclosure relates to a porous membrane, a porous membrane laminate, and a method for manufacturing a porous membrane. This application claims priority from Japanese Patent Application No. 2022-007434 filed on January 20, 2022. All the contents described in the Japanese patent application are incorporated herein by reference.

A porous membrane using polytetrafluoroethylene has the properties of PTFE such as high heat resistance, chemical stability, weather resistance, nonflammability, high strength, non-adhesiveness, and low coefficient of friction, as well as properties such as flexibility due to porosity, dispersion medium permeability, particle trapping, and low dielectric constant. Therefore, porous films containing PTFE as a main component are frequently used as dispersion media and gas precision filters in the semiconductor, liquid crystal, and food and medical fields. As such a filter, in recent years, a porous filter using a porous membrane containing PTFE as a main component capable of trapping fine particles with a particle size of less than 0.1 μm has been proposed (see Japanese Patent Application Laid-Open No. 2010-94579).

JP 2010-94579 A

A porous film according to an aspect of the present disclosure is a porous film containing polytetrafluoroethylene as a main component, and the 1st. The Run melting curve has an endothermic peak in the range of 300° C. to 360° C., and the difference between the onset temperature and the end set temperature of the endothermic peak is 20° C. or less.

FIG. 1 is a schematic partial cross-sectional view showing a porous membrane according to one embodiment of the present disclosure. FIG. 2 is a schematic partial cross-sectional view showing an example of a porous membrane laminate according to an embodiment of the present disclosure. FIG. 3 is a schematic partial cross-sectional view showing another example of the porous membrane laminate according to one embodiment of the present disclosure. FIG. 4 is a schematic partial cross-sectional view showing another example of the porous membrane laminate according to one embodiment of the present disclosure. FIG. 5 is a schematic partial cross-sectional view showing another example of the porous membrane laminate according to one embodiment of the present disclosure. FIG. 6 is a schematic partial cross-sectional view showing another example of the porous membrane laminate according to one embodiment of the present disclosure. FIG. 7 is a schematic partial cross-sectional view showing another example of the porous membrane laminate according to one embodiment of the present disclosure. FIG. 8 is a schematic partial cross-sectional view showing another example of the porous membrane laminate according to one embodiment of the present disclosure. FIG. 9 is a schematic partial cross-sectional view showing another example of the porous membrane laminate according to one embodiment of the present disclosure. FIG. 10 is a schematic partial cross-sectional view showing another example of the porous membrane laminate according to one embodiment of the present disclosure. FIG. 11 is a schematic partial cross-sectional view showing another example of the porous membrane laminate according to one embodiment of the present disclosure. FIG. 12 is a schematic partial cross-sectional view showing another example of the porous membrane laminate according to one embodiment of the present disclosure.

[Problems to be Solved by the Present Disclosure]
As the polytetrafluoroethylene used for the porous membrane, it is preferable to use polytetrafluoroethylene having a high molecular weight in order to reduce the diameter of the pores. However, when the molecular weight of polytetrafluoroethylene increases, the molding pressure increases when molding a material for forming a porous membrane containing polytetrafluoroethylene as a main component, making it difficult to perform extrusion molding using a general-purpose apparatus and manufacturing conditions. In addition, since friction between the material forming the porous membrane and the extruder is likely to occur, the pore size of the porous membrane may vary, and the accuracy of the filtration process may decrease.

The present disclosure has been made based on such circumstances, and aims to provide a porous membrane with small variation in pore size.

[Effect of the present disclosure]
According to the present disclosure, it is possible to provide a porous membrane with small variations in pore size.

[Description of Embodiments of the Present Disclosure]
First, the embodiments of the present disclosure are listed and described.

(1) A porous film according to one aspect of the present disclosure is a porous film containing polytetrafluoroethylene as a main component, and is obtained by differential scanning calorimetry at a temperature increase rate of 10°C/min. The Run melting curve has an endothermic peak in the range of 300° C. to 360° C., and the difference between the onset temperature and the end set temperature of the endothermic peak is 20° C. or less.

The porous membrane is a porous membrane whose main component is polytetrafluoroethylene (hereinafter also referred to as PTFE), and the 1st. Since the difference between the onset temperature (endothermic start temperature) and the endset temperature (endothermic end temperature) of the endothermic peak in the range of 300° C. or higher and 360° C. or lower in the Run melting curve is 20° C. or less, the PTFE particle size has a narrow particle size distribution. If the particle size distribution of the PTFE particles is broad, the gaps between the PTFE particles become small, making it difficult for the liquid lubricant to permeate. On the other hand, the porous membrane has a narrow particle size distribution of the PTFE particle diameter and large gaps between the PTFE particles, so that the liquid lubricant can easily permeate. As a result, the pressure for extruding the material forming the porous membrane is reduced, and the variation in pore size is reduced, so that the accuracy of filtration of the porous membrane can be improved. The above-mentioned "main component" refers to a component having the largest content in terms of mass, for example, a component having a content of 90% by mass or more, preferably 95% by mass or more.

Differential scanning calorimetry is measured using a differential scanning calorimeter (DSC) by the method shown below. 5 mg to 30 mg of the sample was heated from room temperature to 380° C. at a rate of 10° C./min (pattern 1 (1st. Run)), then cooled from 380° C. to 100° C. at a rate of −1° C./min (pattern 2), and then heated from 100° C. to 380° C. at a rate of 10° C./min (pattern 3 (2nd. Run)). Starting from the end set temperature of the endothermic peak in the range of 300° C. or higher and 360° C. or lower in the melting curve of Pattern 1, the endothermic amount obtained by integrating the section of 48° C. is defined as the first heat of fusion. As with the first heat of fusion, the end set temperature of the endothermic peak in the range of 300° C. or higher and 360° C. or lower in the melting curve of Pattern 3 is taken as the starting point, and the endothermic value obtained by integrating the section of 48° C. is defined as the second heat of fusion.

(2) In (1) above, the porous membrane preferably has a difference of 15°C or less between the onset temperature and the endset temperature of the endothermic peak. Thus, by setting the difference between the onset temperature and the endset temperature of the endothermic peak of the porous membrane to 15° C. or less, the extrusion pressure of the material forming the porous membrane is reduced, and the variation in pore size can be further reduced.

(3) In (1) or (2) above, the porous membrane preferably has a porosity of 40% or more and 90% or less. When the porous membrane has a porosity of 40% or more and 90% or less, it is possible to suppress an increase in pressure loss while improving the ability of the porous membrane to capture fine particles. Here, "porosity" means the ratio of the total volume of pores to the volume of the object, and can be obtained by measuring the density of the object according to ASTM-D-792.

(4) In (1) or (2) above, the porous membrane preferably has an average flow pore size of 69 nm or more and 107 nm or less in the pore size distribution. The porous membrane has an average flow pore size of 69 nm or more and 107 nm or less in the pore size distribution, so that it is possible to suppress an increase in pressure loss while improving the performance of trapping fine particles in the porous membrane.

(5) In (1) or (2) above, the porous membrane preferably has a pore size ratio of 17% or more and 49% or less in the pore size distribution. Since the porous membrane has a pore size ratio of 17% or more and 49% or less in the pore size distribution, the pore size variation of the laminate is small, and the accuracy of the filtration process can be improved.

(6) A porous membrane laminate according to another aspect of the present disclosure includes one or more of the porous membranes described in (1) to (5) above. Since the porous membrane laminate includes the porous membrane, the accuracy of the filtration process is excellent, and it is suitable as a microfiltration filter.

(7) In (6) above, the porous membrane laminate according to another aspect of the present disclosure further includes one or more support membranes containing polytetrafluoroethylene as a main component, and the support membrane is preferably laminated on one or both sides of the porous membrane. The porous membrane laminate is provided with one or more porous support membranes, and the membrane is laminated on one or both sides of the porous membrane, so that the support membrane functions as a protective material for the porous membrane. Therefore, the porous membrane laminate can increase the mechanical strength and life of the porous membrane laminate while improving the trapping performance. Moreover, heat resistance, chemical stability, etc. can be improved by using polytetrafluoroethylene as a main component of the support film.

(8) A method for producing a porous membrane according to another aspect of the present disclosure includes a step of forming a kneaded product of polytetrafluoroethylene powder and a liquid lubricant, and the difference between the maximum particle size and the minimum particle size in the primary particle size of the polytetrafluoroethylene is 200 nm or less. The primary particle size referred to here is the minimum unit of polytetrafluoroethylene. The broader the particle size distribution of the primary particles of polytetrafluoroethylene, the narrower the gaps between the PTFE particles, the more difficult it is for the molding aid to permeate, and the lower the extrusion pressure. However, in the method for producing the porous membrane, the difference between the maximum particle size and the minimum particle size in the primary particle size of the polytetrafluoroethylene is 200 nm or less, so that the permeability of the liquid lubricant during molding is excellent, and the extrusion pressure can be reduced. Here, the maximum particle size and the minimum particle size can be analyzed from the particle size distribution of the SEM image obtained by observing the polytetrafluoroethylene powder at 50,000 magnifications using image analysis software Image-Pro.

(9) In (8) above, the 1st. The heat of fusion in the range of 300° C. or higher and 360° C. or lower in the Run melting curve is preferably 60.0 J/g or more. The broader the particle size distribution of the primary particles of polytetrafluoroethylene, the greater the variation in crystallinity inside the particles of polytetrafluoroethylene, and the wider the endothermic peak width of DSC. In the method for producing the porous membrane, the 1st. When the heat of fusion in the range of 300 ° C. to 360 ° C. in the Run melting curve is 60.0 J / g or more, the variation in the crystallinity inside the polytetrafluoroethylene particles is small, so that the liquid lubricant penetration during molding can be further improved and the extrusion pressure can be further reduced.

(10) In (8) above, the 1st. The difference between the onset temperature and the endset temperature of the endothermic peak having a range of 300° C. to 360° C. in the Run melting curve is preferably 20° C. or less. In the method for producing the porous membrane, the 1st. When the difference between the onset temperature and the end-set temperature of the endothermic peak having a range of 300° C. or higher and 360° C. or lower in the Run melting curve is 20° C. or less, the extrusion pressure during the production of the porous membrane can be reduced to a favorable range, and the pore size variation can be reduced.

[Details of the embodiment of the present disclosure]
Preferred embodiments of the present disclosure will be described below with reference to the drawings.

<Porous membrane>
FIG. 1 is a schematic partial cross-sectional view showing a porous membrane according to one embodiment of the present disclosure. The porous membrane 1 is composed of a biaxially stretched porous membrane containing polytetrafluoroethylene as a main component. This biaxially stretched porous membrane is made porous by stretching the surface of a sheet containing PTFE as a main component in two directions perpendicular to each other. The porous membrane 1 allows the filtrate to permeate in the thickness direction while preventing permeation of fine impurities.

　As the PTFE powder, one with a high molecular weight is preferable. The use of high-molecular-weight PTFE powder can promote the growth of the fibrous skeleton while preventing excessive expansion of pores and tearing of the sheet during stretching. In addition, it is possible to reduce the number of nodules in the sheet and form a porous membrane in which fine pores are densely formed.

　As the PTFE powder, one with a high molecular weight is preferable. The use of high-molecular-weight PTFE powder can promote growth of the fibrous skeleton while preventing excessive pore expansion and membrane cleavage during stretching. In addition, it is possible to reduce the number of nodules in the membrane and form a porous membrane in which minute pores are densely formed.

The lower limit of the number average molecular weight of the PTFE powder forming the porous membrane 1 is preferably 12 million, more preferably 20 million. On the other hand, the upper limit of the number average molecular weight of the PTFE powder forming the porous membrane 1 is preferably 50 million, more preferably 40 million. If the number-average molecular weight of the PTFE powder forming the porous membrane 1 is less than the above lower limit, the pore size of the porous membrane 1 may become large and the accuracy of the filtration process may deteriorate. On the other hand, if the number average molecular weight of the PTFE powder forming the porous membrane 1 exceeds the upper limit, formation of the membrane may become difficult. The "number-average molecular weight" is obtained from the specific gravity of the molded product, but the molecular weight of PTFE varies greatly depending on the measurement method and is difficult to measure accurately.

　The 1st. The Run melting curve has an endothermic peak in the range of 300° C. to 360° C., and the difference between the onset temperature and the end set temperature of the endothermic peak is preferably 20° C. or less, more preferably 15° C. or less. When the difference between the onset temperature and the endset temperature of the endothermic peak is 20° C. or less, the particle size distribution of the PTFE particles is narrowed, and the gaps between the particles become large, so that the liquid lubricant can easily permeate. As a result, the extrusion pressure of the forming material of the porous membrane 1 is reduced, and the variation in the pore diameter of the molded porous membrane 1 is reduced, so that the accuracy of the filtration process can be improved. The difference between the onset temperature and the endset temperature of the endothermic peak is preferably as low as possible. The difference between the onset temperature and the endset temperature of the endothermic peak may be 5°C or more and 20°C or less, or 7°C or more and 15°C or less. Porous membrane 1 1st. The onset temperature and endset temperature of the endothermic peak in the range of 300° C. or higher and 360° C. or lower in the Run melting curve can be adjusted, for example, by selecting the molecular weight, crystallinity, primary particle size, etc. of the raw material PTFE.

　The second. The lower limit of the run heat of fusion (second heat of fusion) is preferably 10 J/g, more preferably 14 J/g. On the other hand, the 2nd. The upper limit of the heat of fusion in Run is preferably 23 J/g, more preferably 18 J/g. If the second heat of fusion of the porous membrane 1 is less than the above lower limit, the pore size of the porous membrane 1 becomes large, and there is a risk that the accuracy of the filtration process will decrease. On the other hand, when the second heat of fusion of the porous membrane 1 exceeds the upper limit, formation of the membrane may become difficult.

The lower limit of the average thickness of the porous membrane 1 is preferably 2 µm, more preferably 5 µm. On the other hand, the upper limit of the average thickness of the porous membrane 1 is preferably 50 µm, more preferably 40 µm. If the average thickness is less than the lower limit, the strength of the porous membrane 1 may be insufficient. On the other hand, if the average thickness exceeds the upper limit, the porous membrane 1 becomes unnecessarily thick, which may increase the pressure loss when the filtrate is permeated. When the average thickness of the porous membrane 1 is within the above range, both strength and filtration efficiency of the porous membrane 1 can be achieved. "Average thickness" refers to the average value of ten arbitrary thicknesses, measured using a standard digital thickness gauge.

The upper limit of the average flow pore size in the pore size distribution of the porous membrane 1 is preferably 107 nm or less, more preferably 90 nm or less, and even more preferably 73 nm or less. When the upper limit of the average flow pore diameter of the porous membrane 1 is 107 nm or less, the fine particle trapping performance of the porous membrane 1 is excellent. On the other hand, the lower limit of the average flow pore size in the pore size distribution of the porous membrane 1 is preferably 69 nm or more, more preferably 70 nm or more, and even more preferably 71 nm or more. If the average flow pore diameter of the porous membrane 1 is less than the above lower limit, the pressure loss of the porous membrane 1 may increase. The average flow pore size of the pore size distribution of the porous membrane 1 is preferably 69 nm or more and 107 nm or less, more preferably 70 nm or more and 90 nm or less, and even more preferably 71 nm or more and 73 nm or less. The "average flow pore diameter" can be calculated from the pore diameter distribution measured by a pore diameter distribution measuring device (for example, PMI Perm Porometer "CFP-1500A") in accordance with ASTM F316-03, JIS-K3832: 1990, as described later.

The upper limit of the pore size ratio of the pore size distribution of the porous membrane 1 is preferably 49% or less, more preferably 40% or less, and even more preferably 30% or less. When the upper limit of the pore size ratio of the pore size distribution of the porous membrane 1 is 49% or less, the accuracy of the filtration process can be improved. The lower limit of the pore size ratio of the pore size distribution of the porous membrane 1 is preferably 17% or more, more preferably 18% or more, and even more preferably 19% or more. When the lower limit of the pore size ratio of the pore size distribution of the porous membrane 1 is 17% or more, the pressure loss can be increased. The pore size ratio of the pore size distribution of the porous membrane 1 is preferably 17% or more and 49% or less, more preferably 18% or more and 40% or less, and even more preferably 19% or more and 30% or less. The pore size ratio of the pore size distribution of the porous membrane 1 can be obtained by the method described later.

The upper limit of the porosity of the porous membrane 1 is preferably 90% or less, more preferably 85% or less. On the other hand, the lower limit of the porosity of the porous membrane 1 is preferably 40% or more, more preferably 50% or more. If the porosity of the porous membrane 1 exceeds 90%, there is a risk that the ability of the porous membrane 1 to capture fine particles will be insufficient. On the other hand, if the porosity of the porous membrane 1 is less than 40%, the pressure loss of the porous membrane 1 may increase. The porosity of the porous membrane 1 is preferably 40% or more and 90% or less, more preferably 50% or more and 85% or less.

In addition to PTFE, the porous membrane 1 may contain other fluororesins and additives within a range that does not impair the desired effects of the present disclosure.

[Method for producing porous membrane]
An embodiment of the method for producing the porous membrane will be described. The method for producing the porous membrane includes the steps of forming a kneaded product of PTFE powder and liquid lubricant, and stretching the formed body.

(Process of molding)
In the forming step, a kneaded mixture of PTFE powder produced by emulsion polymerization or the like and a liquid lubricant is extruded to form a sheet. The raw material PTFE particles are powder composed of fine PTFE particles. Examples of the PTFE powder include PTFE fine powder, which is a powder composed of fine particles of PTFE and produced by emulsion polymerization, and PTFE molding powder produced by suspension polymerization.

Various lubricants conventionally used in the extrusion method can be used as the liquid lubricant. Examples of the liquid lubricant include petroleum solvents such as solvent naphtha and white oil, hydrocarbon oils such as undecane, aromatic hydrocarbons such as toluol and xylol, alcohols, ketones, esters, silicone oils, fluorochlorocarbon oils, solutions obtained by dissolving polymers such as polyisobutylene and polyisoprene in these solvents, water or aqueous solutions containing surfactants, and the like. However, from the viewpoint of mixing uniformity, it is preferable to use a single-component liquid lubricant.

The lower limit of the amount of the liquid lubricant mixed with 100 parts by mass of the PTFE powder is preferably 10 parts by mass, more preferably 16 parts by mass. On the other hand, the upper limit of the mixed amount of the liquid lubricant is preferably 40 parts by mass, more preferably 25 parts by mass. If the amount of the liquid lubricant mixed is less than 10 parts by mass, extrusion may become difficult. Conversely, if the amount of the liquid lubricant mixed exceeds 40 parts by mass, compression molding, which will be described later, may become difficult.

The material for forming the porous film may contain other additives in addition to the liquid lubricant, depending on the purpose. Examples of other additives include pigments for coloring, carbon black, graphite, silica powder, glass powder, glass fiber, inorganic fillers such as silicates and carbonates, metal powders, metal oxide powders, and metal sulfide powders for improving wear resistance, preventing cold flow, and facilitating pore formation. In addition, in order to assist the formation of the porous structure, substances that can be removed or decomposed by heating, extraction, dissolution, etc., such as ammonium chloride, sodium chloride, plastics other than PTFE, rubber, etc., may be blended in the form of powder or solution.

In this process, first, the PTFE powder and the liquid lubricant are mixed, and then compression-molded into a block body, which is a primary molded body, by a compression molding machine. Next, this block is extruded into a sheet at a temperature of room temperature (eg, 25° C.) to 50° C. at a speed of, for example, 10 mm/min to 30 mm/min. Furthermore, by rolling this sheet-like body with a calender roll or the like, a PTFE sheet having an average thickness of 250 μm or more and 350 μm or less is obtained.

The difference between the maximum particle size and the minimum particle size in the primary particle size of the polytetrafluoroethylene is 200 nm or less, preferably 190 nm or less. In the method for producing the porous membrane, the difference between the maximum particle size and the minimum particle size in the primary particle size of the polytetrafluoroethylene is 200 nm or less, so that the liquid lubricant has excellent permeability during molding and the extrusion pressure can be reduced.

　In the method for producing the porous membrane, the 1st. The heat of fusion in the range of 300° C. to 360° C. in the Run melting curve is preferably 60.0 J/g or more, more preferably 62.0 J/g or more. The broader the particle size distribution of the primary particles of polytetrafluoroethylene, the greater the variation in crystallinity inside the particles of polytetrafluoroethylene, and the wider the endothermic peak width of DSC. In the method for producing the porous membrane, the 1st. When the heat of fusion in the range of 300 ° C. to 360 ° C. in the Run melting curve is 60.0 J / g or more, the variation in the crystallinity inside the polytetrafluoroethylene particles is small, so that the liquid lubricant penetration during molding can be further improved and the extrusion pressure can be further reduced.

　In the method for producing the porous membrane, the 1st. The difference between the onset temperature and the endset temperature of the endothermic peak having a range of 300° C. to 360° C. in the Run melting curve is preferably 20° C. or less, more preferably 15° C. or less. When the difference between the onset temperature and the endset temperature of the endothermic peak is 20° C. or less, the particle size distribution of the PTFE particles is narrowed, and the gaps between the particles become large, so that the liquid lubricant can easily permeate. As a result, the extrusion pressure of the forming material of the porous membrane 1 is reduced, and the variation in the pore diameter of the molded porous membrane 1 is reduced, so that the accuracy of the filtration process can be improved.

The liquid lubricant contained in the PTFE sheet may be removed after stretching the sheet, but it is preferably removed before stretching. The liquid lubricant can be removed by heating, extraction, dissolution, or the like. When heating, for example, the liquid lubricant can be removed by rolling the PTFE sheet with a hot roll at 130° C. or higher and 220° C. or lower. When using a liquid lubricant having a relatively high boiling point such as silicone oil or fluorochlorocarbon oil, removal by extraction is suitable.

(Step of stretching)
In this step, the PTFE sheet, which is a molded body, is biaxially stretched. Through this step, pores are formed and a porous membrane can be obtained. In this step, a biaxially stretched porous membrane is obtained by sequentially stretching the PTFE sheet in the machine direction (flow direction) and the transverse direction (width direction) perpendicular to the machine direction.

It is preferable to stretch the PTFE sheet at a high temperature in order to make the porous structure dense. The lower limit of the temperature during stretching is preferably 60°C, more preferably 120°C. On the other hand, the upper limit of the temperature during stretching is preferably 300°C, more preferably 280°C. If the temperature during stretching is less than 60°C, the pore size may become too large. Conversely, if the temperature during stretching exceeds 300°C, the pore size may become too small.

Furthermore, the biaxially stretched porous membrane is preferably heat-set after stretching. By performing heat setting, shrinkage of the biaxially stretched porous membrane can be prevented, and the porous structure can be more reliably maintained. As a specific method of heat setting, for example, a method of fixing both ends of the biaxially stretched porous membrane and holding at a temperature of 200° C. or higher and 500° C. or lower for 0.1 minute or longer and 20 minutes or shorter can be used. When stretching is performed in multiple stages, heat setting is preferably performed after each stage.

Since the configuration of the manufactured porous membrane is as described above, redundant description will be omitted.

According to the porous membrane, the extrusion pressure during manufacturing is reduced and the variation in pore size is small, so it can be suitably used as a filter or the like that requires high filtration accuracy.

<Porous membrane laminate>
The porous membrane laminates 10 and 20 include the porous membrane 1 described above. The porous membrane laminate 10, 20 includes one or more porous membranes 1 (FIGS. 2 to 12).

It is preferable that the porous membrane laminate further comprises one or more supporting membranes, and the supporting membranes are laminated on one side or both sides of the porous membrane. This can improve the strength of the porous membrane laminate.

FIG. 2 is a schematic partial cross-sectional view showing a porous membrane laminate according to one embodiment of the present disclosure. A porous membrane laminate 10 shown in FIG. 2 includes a porous membrane 1 and a porous support membrane 2 laminated on one side of the porous membrane 1 . “The porous membrane laminate 10 includes the porous membrane 1 and the porous support film 2 laminated on one side of the porous membrane 1” can be rephrased as “the porous membrane laminate 10 comprises the first porous membrane 1 made of the porous membrane 1 and the first support film 2 mainly composed of polytetrafluoroethylene, and the first porous membrane 1 is arranged on one main surface of the first support film 2.” The porous membrane laminate 10 is provided with the porous support membrane 2 laminated on one side of the porous membrane 1, and the porous membrane 1 is supported by the support membrane 2, so that the mechanical strength can be improved and clogging of the filter can be suppressed.

The porous support film 2 is a porous body, and preferably contains polytetrafluoroethylene as a main component. By using polytetrafluoroethylene as the main component of the support film 2, heat resistance, chemical stability, etc. can be improved.

The upper limit of the average thickness of the support film 2 is preferably 20 µm, more preferably 15 µm. On the other hand, the lower limit of the average thickness of the support film 2 is preferably 2 μm, more preferably 5 μm. If the average thickness of the support membrane 2 exceeds 20 μm, the pressure loss of the porous membrane laminate 10 may increase. On the other hand, if the average thickness of the support film 2 is less than 2 μm, the strength of the porous membrane laminate 10 may be insufficient.

The lower limit of the average flow pore size of the support membrane 2 is preferably 0.08 µm, more preferably 0.10 µm. On the other hand, the upper limit of the mean flow pore size is preferably 3.00 μm, more preferably 1.50 μm. If the average flow pore size of the support membrane 2 is less than 0.08 μm, the pressure loss of the porous membrane laminate 10 may increase. On the other hand, if the average flow pore size of the support membrane 2 exceeds 3.00 μm, the strength of the support membrane 2 may be insufficient.

The porous membrane laminate 20 may have a three-layer structure, and may have a total of three layers, for example, a pair of support membranes 2 arranged as the outermost layers and a porous membrane 1 arranged between the pair of support membranes 2. Also, the porous membrane laminate 20 may have a structure of four or more layers. Examples are shown in FIGS.

It is preferable that the porous membrane laminate includes a plurality of the porous membranes and a plurality of the supporting membranes, the porous membranes and the supporting membranes being alternately laminated, and the supporting membranes being arranged at both ends. In the porous membrane laminate, the porous membrane and the support membrane are alternately laminated, and the support membranes are arranged at both ends, so that the trapping performance, mechanical strength and life of the porous membrane laminate can be further enhanced.

When the porous membrane laminate has, for example, a five-layer structure, it preferably includes a plurality of the porous membranes and a plurality of the supporting membranes, the porous membranes and the supporting membranes being alternately laminated, and the supporting membranes being laminated at both ends. FIG. 6 is a schematic partial cross-sectional view showing a porous membrane laminate according to another embodiment of the present disclosure. The porous membrane laminate 20 shown in FIG. 6 has a five-layer structure in which two layers of the porous membrane 1 are laminated between the pair of support membranes 2 of the outermost layer, and the support membrane 2 is further laminated between the pair of porous membranes 1. In this way, the porous membrane 1 and the support membrane 2 are alternately laminated, and the support membrane 2 is laminated on both ends, so that the trapping performance, mechanical strength and life of the porous membrane laminate 20 can be further enhanced.

For example, as shown in FIG. 3, the porous membrane laminate 10 is composed of a first porous membrane 1 made of the porous membrane 1 and a second porous membrane 1 made of the porous membrane 1, and the second porous membrane 1 can be arranged on one main surface of the first porous membrane 1. As a result, the mechanical strength of the porous membrane laminate 10 can be improved, and the fine particle trapping performance (probability of trapping particles by fibers) can be improved. Here, the first porous film 1 and the second porous film 1 may have the same structure or different structures.

Further, as shown in FIG. 4, for example, the porous membrane laminate 20 includes a first porous membrane 1 made of the porous membrane 1, a first supporting membrane 2 mainly composed of polytetrafluoroethylene, and a second supporting membrane 2 mainly composed of polytetrafluoroethylene. As a result, the mechanical strength of the porous membrane laminate 20 can be improved, filter clogging can be suppressed, and deterioration of particle trapping performance due to trauma can be suppressed. Here, the first support film 2 and the second support film 2 may have the same structure or different structures.

Further, as shown in FIG. 5, for example, the porous membrane laminate 20 is composed of a first porous membrane 1 made of the porous membrane 1, a second porous membrane 1 made of the porous membrane 1, and a first supporting membrane 2 containing polytetrafluoroethylene as a main component. As a result, the mechanical strength of the porous membrane laminate 20 can be improved, filter clogging can be suppressed, and the fine particle trapping performance (probability of trapping particles by fibers) can be improved. Here, the first porous film 1 and the second porous film 1 may have the same structure or different structures.

Further, as shown in FIG. 6, for example, the porous membrane laminate 20 comprises a first porous membrane 1 made of the porous membrane 1, a second porous membrane 1 made of the porous membrane 1, a first supporting membrane 2 mainly composed of polytetrafluoroethylene, and a second supporting membrane 2 mainly composed of polytetrafluoroethylene. 1 and the second support film 2 can be arranged in the above order. As a result, the mechanical strength of the porous membrane laminate 20 can be improved, filter clogging can be suppressed, the fine particle trapping performance (probability of trapping particles by fibers) can be improved, and deterioration of the particle trapping performance due to trauma can be suppressed. Here, the first porous film 1 and the second porous film 1 may have the same structure or different structures. Further, the first support film 2 and the second support film 2 may have the same structure or different structures.

Further, as shown in FIG. 7, for example, the porous membrane laminate 20 comprises a first porous membrane 1 made of the porous membrane 1, a second porous membrane 1 made of the porous membrane 1, a third porous membrane 1 made of the porous membrane 1, and a first support membrane 2 containing polytetrafluoroethylene as a main component. 1 and the third porous membrane 1 can be arranged in the above order. As a result, the mechanical strength of the porous membrane laminate 20 can be improved, filter clogging can be suppressed, and the fine particle trapping performance (probability of trapping particles by fibers) can be improved. Here, the first porous film 1, the second porous film 1, and the third porous film 1 may have the same configuration or different configurations.

Further, as shown in FIG. 8, for example, the porous membrane laminate 20 is composed of a first porous membrane 1 composed of the porous membrane 1, a second porous membrane 1 composed of the porous membrane 1, a first supporting membrane 2 mainly composed of polytetrafluoroethylene, a second supporting membrane 2 mainly composed of polytetrafluoroethylene, and a third supporting membrane 2 mainly composed of polytetrafluoroethylene. The first porous membrane 1, the second supporting membrane 2, the second porous membrane 1, and the third supporting membrane 2 can be arranged in the above order. As a result, the mechanical strength of the porous membrane laminate 20 can be improved, filter clogging can be suppressed, and deterioration of particle trapping performance due to trauma can be suppressed. Here, the first porous film 1 and the second porous film 1 may have the same structure or different structures. Further, the first supporting film 2, the second supporting film 2, and the third supporting film 2 may have the same structure or different structures.

Further, as shown in FIG. 9, for example, the porous membrane laminate 20 is composed of a first porous membrane 1 composed of the porous membrane 1, a second porous membrane 1 composed of the porous membrane 1, a third porous membrane 1 composed of the porous membrane 1, a first supporting membrane 2 mainly composed of polytetrafluoroethylene, and a second supporting membrane 2 mainly composed of polytetrafluoroethylene. The first porous membrane 1, the second porous membrane 1, the third porous membrane 1, and the second support membrane 2 can be arranged in the above order. As a result, the mechanical strength of the porous membrane laminate 20 can be improved, filter clogging can be suppressed, the fine particle trapping performance (probability of trapping particles by fibers) can be improved, and deterioration of the particle trapping performance due to trauma can be suppressed. Here, the first porous film 1, the second porous film 1, and the third porous film 1 may have the same configuration or different configurations. Further, the first support film 2 and the second support film 2 may have the same structure or different structures.

Further, as shown in FIG. 10, for example, the porous membrane laminate 20 comprises a first porous membrane 1 made of the porous membrane 1, a first support film 2 mainly composed of polytetrafluoroethylene, a second support film 2 mainly composed of polytetrafluoroethylene, a third support film 2 mainly composed of polytetrafluoroethylene, and a fourth support film 2 mainly composed of polytetrafluoroethylene. The second support film 2, the first porous film 1, the third support film 2, and the fourth support film 2 can be arranged in the above order. As a result, the mechanical strength of the porous membrane laminate 20 can be improved, filter clogging can be suppressed, and deterioration of particle trapping performance due to trauma can be suppressed. Here, the first supporting film 2, the second supporting film 2, the third supporting film 2, and the fourth supporting film 2 may have the same configuration or different configurations.

Further, as shown in FIG. 11, for example, the porous membrane laminate 20 includes a first porous membrane 1 composed of the porous membrane 1, a second porous membrane 1 composed of the porous membrane 1, a third porous membrane 1 composed of the porous membrane 1, a first supporting membrane 2 composed mainly of polytetrafluoroethylene, a second supporting membrane 2 composed mainly of polytetrafluoroethylene, a third supporting membrane 2 composed mainly of polytetrafluoroethylene, and a polytetrafluoroethylene. and a fourth support film 2 containing fluoroethylene as a main component, wherein the first porous film 1, the second support film 2, the second porous film 1, the third support film 2, the third porous film 1, and the fourth support film 2 are arranged in the above order on one main surface of the first support film 2. As a result, the mechanical strength of the porous membrane laminate can be improved, filter clogging can be suppressed, and deterioration of particle trapping performance due to trauma can be suppressed. Here, the first porous film 1, the second porous film 1, and the third porous film 1 may have the same configuration or different configurations. Further, the first supporting film 2, the second supporting film 2, the third supporting film 2, and the fourth supporting film 2 may have the same configuration or different configurations.

Further, as shown in FIG. 12, for example, the porous membrane laminate 20 includes a first porous membrane 1 composed of the porous membrane 1, a second porous membrane 1 composed of the porous membrane 1, a third porous membrane 1 composed of the porous membrane 1, a fourth porous membrane 1 composed of the porous membrane 1, a fifth porous membrane 1 composed of the porous membrane 1, a first support membrane 2 composed mainly of polytetrafluoroethylene, and polytetrafluoroethylene. and a second support film 2 containing fluoroethylene as a main component, wherein the first porous film 1, the second porous film 1, the third porous film 1, the fourth porous film 1, the fifth porous film 1, and the second support film 2 are arranged in the above order on one main surface of the first support film 2. As a result, the mechanical strength of the porous membrane laminate 20 can be improved, filter clogging can be suppressed, the fine particle trapping performance (probability of trapping particles by fibers) can be improved, and deterioration of the particle trapping performance due to trauma can be suppressed. Here, the first porous film 1, the second porous film 1, the third porous film 1, the fourth porous film 1, and the fifth porous film 1 may have the same configuration or different configurations. Further, the first support film 2 and the second support film 2 may have the same structure or different structures.

[Manufacturing method of porous membrane laminate]
An embodiment of a method for producing a porous membrane laminate including, for example, the porous membrane and a support membrane will be described. The method for manufacturing the porous membrane laminate includes a step of laminating the porous membrane.

(Lamination process)
In this step, the porous membrane laminate is formed by, for example, laminating the porous membrane on one side of the support membrane and heating them.

Examples of the method of laminating the porous membrane on the support membrane include a method of fusion bonding by heating, a method of bonding using an adhesive or a pressure-sensitive adhesive, and the like.

Specifically, the porous membrane is first laminated on one side of a support film, for example, and this laminate is heated to thermally fuse and integrate each layer at the boundary to obtain a porous membrane laminate. The lower limit of the heating temperature is preferably 327°C, which is the glass transition point of PTFE, and more preferably 360°C. On the other hand, the upper limit of the heating temperature is preferably 400°C. If the heating temperature is less than 327° C., there is a possibility that the layers will be insufficiently heat-sealed. On the other hand, if the heating temperature exceeds 400°C, each layer may be deformed. Moreover, the heating time is preferably 0.5 minutes or more and 3 minutes or less.

From the viewpoint of heat resistance, chemical resistance, etc., fluororesin or fluororubber having solvent solubility or thermoplasticity is preferable as the adhesive or adhesive in the method of bonding using an adhesive or adhesive.

(hydrophilic treatment)
Hydrophilization treatment may be performed on the porous membrane laminate obtained as described above. In this hydrophilization treatment, the porous membrane laminate is impregnated with a hydrophilic material and crosslinked. Examples of the hydrophilic material include polyvinyl alcohol (PVA), ethylene vinyl alcohol copolymer (EVOH), acrylate resin, and the like. Among these, PVA is preferable because it is easily adsorbed on the fiber surface of PTFE and can be uniformly impregnated.

Specifically, the hydrophilization treatment can be performed, for example, by the following procedure. First, the porous membrane laminate is immersed in isopropyl alcohol (IPA) for 0.25 to 2 minutes, and then immersed in an aqueous PVA solution having a concentration of 0.5 to 0.8 mass % for 5 to 10 minutes. After that, the porous membrane laminate is immersed in pure water for 2 minutes or more and 5 minutes or less, and then crosslinking is performed by adding a crosslinking agent or irradiating electron beams. After the cross-linking, the porous membrane laminate is washed with pure water and dried at room temperature (25° C.) or higher and 80° C. or lower to make the surface of the porous membrane laminate hydrophilic. As the cross-linking agent, for example, one that forms glutaraldehyde cross-linking, terephthalaldehyde cross-linking, or the like is used. Further, as the electron beam, for example, an electron beam of 6 Mrad can be used.

According to the porous membrane laminate, by providing one or a plurality of the porous membranes, the accuracy of filtration processing is excellent. In addition, according to the porous membrane laminate, one or a plurality of supporting membranes containing polytetrafluoroethylene as a main component are further provided, and the supporting membrane is laminated on one or both sides of the porous membrane, so that the supporting membrane functions as a protective material for the porous membrane. Therefore, the porous membrane laminate can improve the trapping performance and increase the mechanical strength and life of the porous membrane laminate. Moreover, heat resistance, chemical stability, etc. can be improved by using polytetrafluoroethylene as a main component of the support film. Therefore, it is suitable for a dispersion medium and gas precision filtration filter used for cleaning, peeling, chemical supply, etc. in the semiconductor-related field, the liquid crystal-related field, and the food and medical-related field.

[Other embodiments]
It should be considered that the embodiments disclosed this time are illustrative in all respects and not restrictive. The scope of the present invention is not limited to the configurations of the above-described embodiments, but is indicated by the scope of the claims, and is intended to include all modifications within the scope and meaning equivalent to the scope of the claims.

[Appendix 1]
A porous membrane containing polytetrafluoroethylene as a main component,
1st. obtained by differential scanning calorimetry at a heating rate of 10° C./min. The melting curve of Run has an endothermic peak in the range of 300 ° C. or higher and 360 ° C. or lower,
The porous membrane, wherein the difference between the onset temperature and the endset temperature of the endothermic peak is 20°C or less.

[Appendix 2]
The porous membrane according to appendix 1, wherein the difference between the onset temperature and the endset temperature of the endothermic peak is 15° C. or less.

[Appendix 3]
The porous membrane according to appendix 1 or appendix 2, which has a porosity of 40% or more and 90% or less.

[Appendix 4]
The porous membrane according to appendix 1 or appendix 2, wherein the mean flow pore size of the pore size distribution is 69 nm or more and 107 nm or less.

[Appendix 5]
The porous membrane according to appendix 1 or appendix 2, wherein the pore size ratio in the pore size distribution is 17% or more and 49% or less.

[Appendix 6]
A porous membrane laminate comprising one or a plurality of porous membranes according to any one of Appendixes 1 to 5.

[Appendix 7]
7. The porous membrane laminate according to appendix 6, further comprising one or a plurality of supporting films containing polytetrafluoroethylene as a main component, wherein the supporting films are laminated on one side or both sides of the porous membrane.

[Appendix 8]
A step of forming a kneaded product of polytetrafluoroethylene powder and a liquid lubricant,
A method for producing a porous membrane, wherein the difference between the maximum particle size and the minimum primary particle size of the polytetrafluoroethylene is 200 nm or less.

[Appendix 9]
1 st. 9. The method for producing a porous membrane according to appendix 8, wherein the heat of fusion in the range of 300° C. or higher and 360° C. or lower in the Run melting curve is 60.0 J/g or more.

[Appendix 10]
1 st. 9. The method for producing a porous membrane according to appendix 8, wherein the difference between the onset temperature and the endset temperature of the endothermic peak having a range of 300° C. or more and 360° C. or less in the Run melting curve is 20° C. or less.

[Appendix 11]
A first porous film made of the porous film according to any one of Appendices 1 to 5, and a second porous film made of the porous film according to any one of Appendices 1 to 5,
A porous membrane laminate in which the second porous membrane is arranged on one main surface of the first porous membrane.

[Appendix 12]
A first porous film made of the porous film according to any one of Appendices 1 to 5, a first support film containing polytetrafluoroethylene as a main component, and a second support film containing polytetrafluoroethylene as a main component,
A porous membrane laminate in which the first porous membrane and the second supporting membrane are arranged in the order described above on one main surface of the first supporting membrane.

[Appendix 13]
A first porous film made of the porous film according to any one of Appendices 1 to 5, a second porous film made of the porous film according to any one of Appendices 1 to 5, and a first support film containing polytetrafluoroethylene as a main component,
A porous membrane laminate in which the first porous membrane and the second porous membrane are arranged in the order described above on one main surface of the first supporting membrane.

[Appendix 14]
A first porous film made of the porous film according to any one of Appendices 1 to 5, a second porous film made of the porous film according to any one of Appendices 1 to 5, a first support film containing polytetrafluoroethylene as a main component, and a second support film containing polytetrafluoroethylene as a main component,
A porous membrane laminate in which the first porous membrane, the second porous membrane, and the second supporting membrane are arranged in the order described above on one main surface of the first supporting membrane.

[Appendix 15]
A first porous film made of the porous film according to any one of Appendices 1 to 5, a second porous film made of the porous film according to any one of Appendices 1 to 5, a third porous film made of the porous film according to any one of Appendices 1 to 5, and a first support film containing polytetrafluoroethylene as a main component,
A porous membrane laminate in which the first porous membrane, the second porous membrane, and the third porous membrane are arranged in the order described above on one main surface of the first supporting membrane.

[Appendix 16]
A first porous film made of the porous film according to any one of Appendices 1 to 5; a second porous film made of the porous film according to any one of Appendices 1 to 5; a first support film containing polytetrafluoroethylene as a main component; a second support film containing polytetrafluoroethylene as a main component;
A porous membrane laminate in which the first porous membrane, the second supporting membrane, the second porous membrane, and the third supporting membrane are arranged in the order described above on one main surface of the first supporting membrane.

[Appendix 17]
A first porous film made of the porous film according to any one of Appendices 1 to 5; a second porous film made of the porous film according to any one of Appendices 1 to 5; a third porous film made of the porous film according to any one of Appendices 1 to 5;
A porous membrane laminate in which the first porous membrane, the second porous membrane, the third porous membrane, and the second supporting membrane are arranged in the order described above on one main surface of the first supporting membrane.

[Appendix 18]
A first porous film made of the porous film according to any one of Appendices 1 to 5, a first support film containing polytetrafluoroethylene as a main component, a second support film containing polytetrafluoroethylene as a main component, a third support film containing polytetrafluoroethylene as a main component, and a fourth support film containing polytetrafluoroethylene as a main component,
A porous membrane laminate in which the second supporting membrane, the first porous membrane, the third supporting membrane, and the fourth supporting membrane are arranged in the order described above on one main surface of the first supporting membrane.

[Appendix 19]
A first porous film comprising the porous film according to any one of Appendices 1 to 5; a second porous film comprising the porous film according to any one of Appendices 1 to 5; a third porous film comprising the porous film according to any one of Appendices 1 to 5; and a fourth support film mainly composed of polytetrafluoroethylene,
A porous membrane laminate in which the first porous membrane, the second supporting membrane, the second porous membrane, the third supporting membrane, the third porous membrane, and the fourth supporting membrane are arranged in the order described above on one main surface of the first supporting membrane.

[Appendix 20]
A first porous film comprising the porous film according to any one of Appendices 1 to 5; a second porous film comprising the porous film according to any one of Appendices 1 to 5; a third porous film comprising the porous film according to any one of Appendices 1 to 5; a fourth porous film comprising the porous film according to any one of Appendices 1 to 5; A fifth porous film made of the porous film according to item 1, a first support film containing polytetrafluoroethylene as a main component, and a second support film containing polytetrafluoroethylene as a main component,
A porous membrane laminate in which the first porous membrane, the second porous membrane, the third porous membrane, the fourth porous membrane, the fifth porous membrane, and the second supporting membrane are arranged in the order described above on one main surface of the first supporting membrane.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

<Porous membrane>
[Test No. 1]
(Molding process)
PTFE fine powder A (second heat of fusion: 15.8 J/g, molecular weight: about 28,000,000) shown in Table 1 was used as the raw material powder. The PTFE fine powder A used here is a powder obtained by drying and granulating PTFE particles (primary particles) produced by emulsion polymerization of tetrafluoroethylene (emulsion polymerization product). Table 1 shows the specific gravity of PTFE fine powder A and the results of differential scanning calorimetry using a differential scanning calorimeter (“DSC-60A” manufactured by Shimadzu Corporation). In addition, differential scanning calorimetry by a differential scanning calorimeter was performed by the above-mentioned method.

18 parts by mass of naphtha (“Supersol FP-25” manufactured by Idemitsu Oil Co., Ltd., boiling point: 50° C. to 180° C.) as a liquid lubricant was kneaded with 100 parts by mass of PTFE fine powder. Next, the kneaded material was placed in a molding machine and compression molded to obtain a block-shaped molded product (primary molded product). Next, the block-shaped molding was continuously extruded into a sheet, passed through a rolling roller, passed through a heating roll (130° C. to 220° C.) to remove the liquid lubricant, and wound around a roll to form a PTFE sheet having an average thickness of 320 μm. Next, the film was stretched 4 times in the machine direction (machine direction) at a roll temperature of 250°C to 280°C. Subsequently, both ends of the longitudinally stretched film in the lateral direction (width direction) were gripped with chucks, and the film was stretched 25 times in an atmosphere of 150 ° C. in the lateral direction, which is the direction perpendicular to the machine direction. By this stretching, test no. A porous membrane of No. 1 was obtained.
The sheet thus stretched was passed through a heating furnace at 360° C. and sintered for 1.5 minutes. A porous membrane of No. 1 was obtained.

[Test No. 2]
PTFE fine powder B (second heat of fusion: 17.0 J/g, molecular weight: about 23,000,000) having a specific gravity and differential scanning calorimetry results shown in Table 1 was used as the raw material powder, and 100 parts by mass of the PTFE fine powder was kneaded with 16 parts by mass of naphtha as a liquid lubricant. Test No. 1 by the same process as the porous membrane of Test No. 1. No. 2 porous membrane was obtained.

[Test No. 3]
PTFE fine powder C (second heat of fusion 16.8 J/g, molecular weight of about 22 million) having a specific gravity and differential scanning calorimetry results shown in Table 1 was used as the raw material powder, and 100 parts by mass of PTFE fine powder was kneaded with 16 parts by mass of naphtha as a liquid lubricant. Test No. 1 by the same process as the porous membrane of Test No. 1. No. 3 porous membrane was obtained.

<Evaluation>
[Extrusion pressure]
Using a capillary rheometer ("RH7" manufactured by Malvern), measurement was performed by the method shown below. 20 parts by mass of isoparaffinic hydrocarbon (Idemitsu Supersol FP-25) was used as a liquid lubricant and kneaded with 100 parts by mass of PTFE fine powders A, B and C. Next, the kneaded material was filled in a barrel portion having a diameter of 15 mm, and was extruded at a measurement temperature of 50° C. at a speed of 100 mm/min using a capillary die, and the pressure was measured. A capillary die with an inlet angle of 90° and a capillary die length of 0.25 mm and an inner diameter of 2.0 mm was used.

[Average flow pore size and pore size ratio]
First, test no. 1 to test No. For the porous membrane No. 3, propylene, 1,1,2,3,3,3 hexafluoric acid with a surface tension of 15.9 mN/m (“GALWICK” manufactured by PMI) was used as a reagent in accordance with ASTM F316-03 and JIS-K3832: 1990, and the pore size distribution was measured using a pore size distribution measuring device (perm porometer “CFP-1500A” manufactured by PMI). Then, the maximum pore diameter [nm] and the average flow pore diameter [nm] were obtained from the pore diameter distribution, and the pore diameter ratio [%] was calculated from the following formula. The larger the pore diameter ratio, the greater the variation in the pore diameters of the porous membrane, which means that the accuracy of the filtration treatment of the porous membrane is lowered.
Pore diameter ratio [%] = {(maximum pore diameter - average flow pore diameter) / average flow pore diameter} x 100

[Porosity]
Porosity is determined by test No. according to ASTM-D-792. 1 to test No. The density of the porous membrane of No. 3 was measured, and the ratio of the total volume of pores to the volume of each porous membrane was obtained. Specifically, the porosity was calculated by the following procedure. First, test no. 1 to test No. 3 was punched into a circle of Φ60 mm to prepare a sample, and then the mass [g] and thickness [mm] were measured. Mass was measured using a mass balance and thickness was measured using a digital thickness gauge. Then, the porosity [%] was calculated from the following formula.
Porosity [%] = [1-{mass / (area of one side of sample x thickness x specific gravity 2.17)}] x 100

"Test No. 1 to test No. Table 1 shows the evaluation results of the extrusion pressure, pore diameter ratio and porosity of the porous membrane No. 3.

As shown in Table 1, the 1st. Test No. in which the difference between the onset temperature and the endset temperature of the endothermic peak having a range of 300° C. or higher and 360° C. or lower in the Run melting curve is 20° C. or lower. 1 and test no. The porous membrane of Test No. 2 was tested. The extrusion pressure and pore size ratio were lower than the porous membrane of No. 3. From this, Test No. 1 and test no. The porous membrane of No. 2 has a small variation in pore diameter and has high-precision filtration performance. In particular, test no. In the porous membrane of No. 1, 2nd. Although Run has a low heat of fusion and PTFE fine powder A has the highest molecular weight, 1st. When the difference between the onset temperature and the endset temperature of the endothermic peak in the range of 300° C. or higher and 360° C. or lower in the Run melting curve is 20° C. or less, the extrusion pressure during the production of the porous membrane is reduced to a favorable range, and the variation in pore size is also reduced.
Also, test no. 1 and test no. 2 has a difference of 200 nm or less between the maximum particle size and the minimum particle size in the primary particle size, and is obtained by differential scanning calorimetry at a heating rate of 10° C./min. By using PTFE fine powder with a heat of fusion of 60.0 J/g or more in the range of 300° C. or higher and 360° C. or lower in the melting curve of Run as a raw material, it is believed that the permeability of the liquid lubricant during molding was further improved, and the extrusion pressure could be further reduced.

From the above results, the porous membrane can be suitably used as a filter or the like that requires high-precision filtration performance, because the extrusion pressure during manufacturing is reduced and the variation in pore size is small.

1 Porous membrane, 2 Supporting membrane, 10, 20 Porous membrane laminate.

Claims (10)

1. A porous membrane containing polytetrafluoroethylene as a main component,
   1st. obtained by differential scanning calorimetry at a heating rate of 10° C./min. The melting curve of Run has an endothermic peak in the range of 300 ° C. or higher and 360 ° C. or lower,
   The porous membrane, wherein the difference between the onset temperature and the endset temperature of the endothermic peak is 20°C or less.
2. The porous membrane according to claim 1, wherein the difference between the onset temperature and the endset temperature of the endothermic peak is 15°C or less.
3. The porous membrane according to claim 1 or claim 2, which has a porosity of 40% or more and 90% or less.
4. The porous membrane according to claim 1 or 2, wherein the average flow pore size of the pore size distribution is 69 nm or more and 107 nm or less.
5. The porous membrane according to claim 1 or claim 2, wherein the pore size ratio of the pore size distribution is 17% or more and 49% or less.
6. A porous membrane laminate comprising one or a plurality of porous membranes according to any one of claims 1 to 5.
7. The porous membrane laminate according to claim 6, further comprising one or more supporting membranes containing polytetrafluoroethylene as a main component, and the supporting membranes being laminated on one side or both sides of the porous membrane.
8. A step of forming a kneaded product of polytetrafluoroethylene powder and a liquid lubricant,
   A method for producing a porous membrane, wherein the difference between the maximum particle size and the minimum primary particle size of the polytetrafluoroethylene is 200 nm or less.
9. obtained by differential scanning calorimetry at a temperature elevation rate of 10°C/min for the polytetrafluoroethylene, the 1st. 9. The method for producing a porous membrane according to claim 8, wherein the heat of fusion in the range of 300° C. or higher and 360° C. or lower in the Run melting curve is 60.0 J/g or more.
10. obtained by differential scanning calorimetry at a temperature elevation rate of 10°C/min for the polytetrafluoroethylene, the 1st. 9. The method for producing a porous membrane according to claim 8, wherein the difference between the onset temperature and the endset temperature of the endothermic peak having a range of 300° C. or more and 360° C. or less in the Run melting curve is 20° C. or less.